The disposal of both industrial and domestic solids such as trash, rubbish, garbage, etc. is becoming an immense national problem. The cost of this service presently ranks third behind public schooling and highways as a municipal expense in the United States. The cost per unit of trash disposal and the number of units of trash per person are rising annually. It is estimated that each individual in the country generates between 4-6 pounds per day of solid waste, and that the industrial output is equivalent to approximately five pounds of solid waste per person per day. The cost of disposal varies from 5 to 30 dollars per ton of trash. Previous methods of trash disposal, such as land fill, are becoming impossible, while others such as incineration are costly and result in air pollution problems. Less costly and more efficient disposal means for solid waste appear mandatory.
A second aspect of this problem is that the United States is consuming its natural resources at an ever increasing rate. In the normal materials utilization cycle, raw materials are collected, processed into useful products, utilized by consumers for varying spans of time, and then consigned to a presumably uncoverable wasteland, the city dump.
Because of these problems, many proposals have been put forth to utilize and recover values from solid wastes. Aluminum companies and glass companies will purchase used cans and bottles for reprocessing. Engineering studies and plant designs have been prepared to advance the concept of utilizing the heat produced by garbage incineration to operate electrical and desalination plants.
The idea of recovering metal values from waste solids is old in the art and is an integral part of the steel making industry.
However, the art must now develop processes to utilize both the metallic and non-metallic portion of waste solids as a raw material since these represent a large portion of the waste solids. Simple incineration of the organic portion of waste solids to produce utilizeable heat is not the solution for several reasons. The off gases produced during incineration contain air pollutants, such as SO.sub.2, NO.sub.x, CO and ash. These pollutants must be trapped or diminished which requires costly devices such as electrostatic precipitators, scrubbers, etc to avoid air pollution. In addition, organic waste solids are a poor fuel, and require very high combustion temperatures. What is needed is an efficient, economical method for handling the conventional waste solids produced by society which will recover chemical and fuel values from both the inorganic and organic portions of waste solids while substantially reducing the volume of gaseous effluent which must be treated to eliminate air pollution during processing.
The goal of totally recycling the raw materials contained in municipal solid wastes has become almost a "holy grail" to many of the young members of our changing society. Although the idea is old, it was they who dramatized the quest, and when Congress passed the Solid Waste Disposal Act of 1965, the American people set their sights on the same goal. More recently, when Congress passed the Resource Recovery Act of 1970, the goal was more clearly defined and the quest may now receive significant taxpaper support. The end result should not only be a beautification of the American scene, but also a reduction in the financial drain on the taxpayer who is now asked to contribute toward the achievement of this goal. The present financial drain is truly staggering. In 1968, about $4.5 billion was spent by municipalities to collect, and either bury or burn our solid wastes. If something is not done to change waste elimination procedures,, the cost estimates for 1980 range from $12.5 to $16.5 billion. Although about three quarters of these costs go for antiquated and difficult to change collection procedures, there is hope to significantly reduce, or even eliminate, the current disposal costs to the urban community.
At the present time, a significant amount of discarded raw materials is being recycled to the economy by many companies engaged in America's secondary materials industry. Large quantities of metals, an appreciable amount of paper, and some glass is being collected, upgraded and reused. However, except for tin and aluminum cans in some scattered areas of the nation, only a small fraction of our reuseable resources are being recovered once they enter the municipal collection stream. A typical breakdown of municipal refuse is shown in the following Table 1, and up to now the difficult problem has been how to separate the vast amount of contaminated materials from the heterogeneous mass, and recover the potential values shown in this Table. In the past few years, American industry has tackled this problem, and answers are indeed beginning to come forth.
TABLE 1 ______________________________________ RECOVERABLE MATERIALS IN MUNICIPAL SOLID WASTES Estimated Recovery Composition Eff. Raw Material Wt. % % ______________________________________ Group 1 Magnetic metals 6 - 8 95 Non-magnetic metals 1 - 2 95 Glass 6 - 10 80 Dirt and debris 2 - 4 0 Subtotal Group 2 Paper products 48 - 55 50 Group 3 Unreclaimed paper and other organics 55 100 ______________________________________
We have invented a process which overcomes the above problems. The key to our process lies in converting the unuseable organic portion of these wastes to cleaning-burning, low-sulfur heating fuels using an efficient, low-cost, high capacity pyrolysis operation. Over one barrel of good quality, liquid heating fuel can be obtained per ton of wet as-received municipal refuse. This process has been researched on a small continuous bench scale unit and in a pilot plant.
Our novel pyrolysis process is based upon the heating of shredded organic waste materials in the absence of air using a novel heat-exchange system. This method was developed to maximize liquid yields and thus generate the maximum liquid chemical and fuel value per ton of wastes. At the present time, organic chemical and fuel liquids yields of greater than 40 weight percent are being obtained from oven-dried, inorganic-free feed material obtained from typical as-delivered municipal solid waste. This liquid has an average heating value of from about 9000 to about 12,000 Btu per pound and can be used as a low-sulfur replacement for No. 6 fuel oil. Pyrolysis of organic waste materials also produce char, gases, and a water fraction. The distribution of these products is the most important economic factor involved in commercial pyrolysis equipment. Most other prior art units produce relatively little organic liquids unless high pressure hydrogenation is employed. An example of the distribution and analysis of the products which can be obtained in our atmospheric pressure pyrolysis process is shown in the following Table 2.
TABLE 2 ______________________________________ PRODUCTS OF PYROLYSIS Char fraction, 35 wt. %; Heating value 9,000 Btu/lb 48.8 wt. % Carbon 3.9 Hydrogen 1.1 Nitrogen 0.3 Sulfur 31.8 Ash 0.2 Chlorine 12.7 Oxygen (by difference) Oil fraction, 40 wt. %; Heating value 12,000 Btu/lb 60.0 wt. % Carbon 8.0 Hydrogen 1.0 Nitrogen 0.2 Sulfur 0.4 Ash 0.3 Chlorine 20.0 Oxygen (by difference) Gas fraction, 10 wt. %; Heating value 600 Btu/cu. ft. 0.1 mol % Water 42.0 Carbon monoxide 27.0 Carbon dioxide 10.5 Hydrogen &lt;0.1 Methyl chloride 5.9 Methane 4.5 Ethane 8.9 C.sub.3 to C.sub.7 hydrocarbons Water fraction, 15 wt. % Contains: Acetaldehyde Methanol Acetone Methylfurfural Formic Acid Phenol Furfural Etc. ______________________________________
Aside from the organic liquid, or oil yields of 40 wt. % obtained in a typical run, about 35% char, 10% gases and 15% water and also obtained. The gases and some of the char are used for a heat source in carrying out the process, and the oil and remaining char can be sold as a fuel or raw chemical.
The pyrolysis process is flexible with regard to feed materials. So far, the following waste products have been converted to useful chemical and fuel liquids and chars: tree bark, rice hulls, animal feed lot wastes, and shredded automobile tires. In the case of tires, the char produced is recyclable into new tire manufacturing as carbon black. Tests conducted on this product shows that modulus of elasticity and tensile strength of the compounded rubber approach to within 75 to 85% of the properties obtained when general purpose carbon black is used.